1. Field of the Invention
The present invention relates to a gas concentration detector for detecting a specific gas component in a gas to be measured by using a plurality of cells provided for in a solid electrolyte body. More specifically, the detector can be used for detecting an NOx concentration contained in the exhaust gas from an internal combustion engine of a vehicle.
2. Description of the Related Art
Generally, concern over the global environment is increasing. Correspondingly, the regulations on the exhaust gas from internal combustion engines for vehicles is becoming stricter year by year. In response to these regulations, more precise control of exhaust gas is expected. That is, the detection of harmful materials in the exhaust gas is expected, for example, detection of NOx concentrations, to feedback the result of detection to an EGR (Exhaust Gas Recirculation) system, a catalyst system. and the like.
As such, a gas concentration detector using a plurality of cells provided for an oxygen ion conductive solid electrolyte body to detect the concentration of NOx by taking advantage of a difference in activity to NOx reduction has been known. For example, see Japanese Patent Laid-Open Publication No. Hei 9-288086 (1997). Such a conventional gas concentration detector generally has a pump cell for exhausting and pumping oxygen into an exhaust gas introduced into a chamber, a monitor cell for generating an output in accordance with a concentration of oxygen remaining in the chamber, and a sensor cell for generating an output in accordance with the concentrations of remaining oxygen and NOx in the chamber. For example, pump cell voltage is feedback-controlled so that the oxygen concentration in the chamber, which is detected by the monitor cell, is kept constant. At the same time, an NOx concentration in the exhaust gas is detected based on a current value of a current flowing through the sensor cell.
Moreover, a space in the chamber is normally partitioned into a first chamber where the pump cell is placed and a second chamber where the sensor cell and the monitor cell are placed. These first and second chambers are connected to each other through a throttle. With this configuration, a variation in oxygen concentration in the vicinity of the sensor cell and the monitor cell can be decreased. However, a change in oxygen concentration in the first chamber due to a change in pump cell voltage is not immediately reflected in the oxygen concentration in the second chamber (monitor cell current value). Therefore, there is a possibility that the oxygen concentration in the second chamber may not be stabilized. In view of such a problem, the detection of the NOx concentration in an exhaust gas based on a difference in output between the sensor cell and the monitor cell has been proposed. According to such a detection, the detection accuracy is advantageously improved because a sensor output independent of the oxygen concentration in the second chamber can be obtained.
However, as chamber-side electrodes, the sensor cell uses a Pt—Rh electrode that is active to reduction of any NOx, whereas the monitor cell uses a Pt—Au electrode that is inactive to reduction of any NOx. Therefore, these electrode materials have different reactivities (responses) to oxygen. Such a difference in reactivity results from the fact that Rh contained in the sensor cell electrode has oxygen storage ability so that the sensor cell electrode is more likely to take up oxygen contained in the exhaust gas as compared with the monitor cell. As a result, the sensor cell has a low reactivity to a variation in oxygen concentration. Thus, for example, when the operation condition of an engine is changed to vary an oxygen concentration in the exhaust gas, or when a concentration of oxygen remaining in the second chamber is changed, a deviation is generated between an output current of the sensor cell and that of the monitor cell. As a result, a difference in output between the sensor cell and the monitor cell is varied to change the detected NOx value, adversely preventing accurate detection of NOx.